In the CDMA system, since the same frequency band is shared by multiple users, signals of other users become interference signals which degrade the communication quality of a particular user. When a base station communicates with near and remote mobile stations at the same time, it receives the transmitted signal from the near mobile station at a high level, whereas from the remote one at a much lower level. Thus, the communications between the base station and the remote mobile station presents a problem in that the channel quality is sharply degraded by interference from the near mobile station, which is generally called a near-far problem.
As one of the techniques used for solving the near-far problem, transmission power control has been studied which controls the transmission power such that the received power of a receiving station, or the SIR (Signal-to-Interference plus noise power Ratio) thereof is kept fixed regardless of the location of a mobile station. This will provide consistent channel quality across a service area. In particular, for reverse link channels, the transmission power control of the mobile stations are carried out so that the received power or SIR is kept constant of the signals transmitted from the mobile stations and received at the receiving terminal of the base station.
Since the CDMA scheme considers the interference from other users as white noise, an increasing number of users is equivalent to an increase of noise power. Thus, the capacity in terms of the number of simultaneous users in the same cell depends on the received SIR for achieving required channel quality. In contrast, concerning the forward link channels, since incoming signals from the intended user and other users causing the interference arrive through the same propagation path, their long term fluctuations, short term fluctuations and instantaneous fluctuations are the same, and hence the received SIR except for the noise power is always constant. This means that no transmission power control is necessary as long as the interference within the same cell is handled. However, since the CDMA scheme with its interference equivalent to white noise reuses the same frequency band in contiguous cells, the interference therefrom must be taken into account. Although the interference power from other cells has instantaneous fluctuations due to Rayleigh fading like the interference within the cell, the fluctuations differ from those of the desired signal.
The CDMA system standardized by TIA in the United States does not basically perform the forward link transmission power control. Instead, a method is employed in which a mobile station detects a frame error rate, and requires the base station to increase the transmission power to the mobile station when the error rate exceeds a predetermined threshold value. This is because a sharp increase of the transmission power would increase the interference to other cells. However, since the signals transmitted from other cell base stations become instantaneously fluctuating interference signals, the conventional technique described above cannot follow them.
As a transmission power control method to follow the instantaneous fluctuations, a closed loop transmission power control system is known which employs transmission power control bits. In this control system, when a mobile station within a cell communicates with the base station in that cell, it measures the received SIR of the desired wave from the base station, and determines the transmission power control bits for controlling the transmission power of the base station in accordance with the measurement results. Subsequently, the mobiles station inserts the transmission power control bits into its transmitted signal and transmits them to the base station. Receiving the signal transmitted from the mobile station, the base station extracts the transmission power control bits therefrom, and determines the transmission power in accordance with the command of the transmission power control bits. In addition, the base station measures the received SIR of the wave from the mobile station, and determines the transmission power control bits for controlling the transmission power of the mobile station on the basis of the measurement results. Then, the base station inserts the transmission power control bits into the transmitted signal, and transmits them to the mobile station. Receiving the signal transmitted from the base station, the mobile station extracts the transmission power control bits, and determines the transmission power in accordance with the instructions of the transmission power control bits. The closed loop thus formed between the mobile station and the base station enables the mobile station to control its transmission power. The close loop transmission power control using the transmission power control bits requires a highly accurate SIR measurement technique.
Conventional SIR measurement techniques, however, have a problem in that the measurement accuracy of the received desired wave power can be degraded at the places where the envelope of the received signal falls owing to fading. This is because they perform the measurements using the received data symbols decided by the interpolation of pilot signals, and this can cause large differences between actual and interpolated values of the received data symbols at these places.
In addition, using the entire interval between intermittent pilot signals as the integration interval of the fading envelope power can cause the degradation in the measurement accuracy of the received interference power because of the decision error of signal points of the received signal.
Therefore, an object of the present invention is to provide a received SIR measuring method and apparatus which can improve the accuracy of the received SIR measurement.
Another object of the present invention is to achieve the transmission power control by applying the measuring apparatus.